1. Field of the Invention
The present invention relates to determining thermal properties of materials using a miniaturized resistive thermal probe. More particularly, the present invention relates to performing localized thermal analysis experiments whereby calorimetric information is obtained from a volume of materials on the order of a few cubic microns, whereas in conventional bulk calorimetry data is obtained from volumes of material on the order of a few cubic millimeters. The present invention also relates to modulating the temperature of a thermal probe to generate evanescent thermal waves in a material to thereby generate sub-surface images.
2. Background of the Invention
Several methods for the non-destructive characterization of solids make use of thermal excitation. Any local disruption of the structure that results in a change in density, specific heat, or thermal conductivity may be detected by some type of thermal probe--often with higher sensitivity than by the use of optical, X-ray, or electron-microscope techniques. Many of these techniques use an intensity-modulated energy source to excite a sample. That is, the intensity of the energy source used to excite the sample is made to vary with time. Induced scattered evanescent thermal waves are then detected by, for example, monitoring the surface temperature of the sample. When a scanning mechanism is also incorporated, it is then possible to achieve spatial thermal mapping. Subsurface imaging can be performed within the depth of penetration of the thermal wave.
Most conventional methods of thermal imaging employ an energy beam that emerges from a small source and spreads out according to the rules of diffraction. The extent of this spreading is normally governed by the wavelength associated with the energy flux. However, if the sample is within the "near-field" region, i.e., significantly less than one wavelength away from the source, then a greatly reduced beam diameter can be achieved. In this case, the beam diameter is not much larger than the size of the source itself.
This principle is applied in Scanning Probe Microscopy whereby a sharp probe is brought in close proximity to the surface of a sample. Some probe/sample interaction takes place. This interaction is monitored as the probe is scanned over the surface. An image contrast is then computer-generated. The image contrast represents variations of some property (e.g., physical, mechanical, chemical) of the sample across the scanned area. One such scanning probe microscope is the Atomic Force Microscope (AFM). In conventional AFM, the height of a probe above the surface being scanned is controlled by a feedback system, that keeps the force between the probe and the surface of the sample constant. The probe height is monitored, and provides the data that is used to create image contrast which represents the topography of the scanned area.
Near-field thermal imaging is described by C. C. Williams and H. K. Wickramasinghe in Photoacoustic and Photothermal Phenomena, P. Hess and J. Peltzl (eds.), p. 364 (1988). In their device, the probe is a specially made coaxial tip that forms a fine thermocouple junction. This probe provides a spatial resolution on the order of tens of millimeters. The sample is either heated using a laser or the probe, or the sample is heated electrically. The feedback system maintains the probe temperature constant (instead of maintaining the force constant), by varying the probe height as necessary.
In "Thermal Imaging Using the Atomic Force Microscope," Appl. Phys. Lett., vol. 62, pp. 2501-3 (1993), Majumdar, et al. describe a technique for thermal imaging that uses a simpler design of thermocouple tip, than that disclosed by Williams and Wickramasinghe. Majundar, et al. implemented standard atomic force microscopy feedback to maintain tip/sample contact. R. B. Dinwiddie, R. J. Pylkki and P. E. West "Thermal Conductivity Contrast Imaging with a Scanning Thermal Microscope," Thermal Conductivity 22, T. W. Tong (ed.) (1994), describe the use of a probe in the form of a tiny platinum resistance thermometer. U.S. Pat. No. 5,441,343 to Pylkki et al., which is incorporated herein by reference, discloses the thermal sensing probe for use with a scanning probe microscope, in which the contact force of the probe is maintained at a constant level as the probe is scanned across the surface of the sample.
In those studies, the samples were generally probed at a constant (ac or dc) amplitude of either surface temperature or heat flow. Thus changes in the thermal properties of materials, such as heat capacity or thermal conductivity, were not investigated. This is because the temperature of the sample was not raised by a sufficient amount for a change in the sample's thermal properties to be detected.
Bulk thermal analysis techniques have been developed to study such changes in the thermal properties of materials. Modulated temperature differential scanning microscopy (MDSC) and spatially-resolved modulated differential scanning calorimetry (SR-MDSC) are described in U.S. Pat. No. 5,224,775 ('775 patent) and U.S. Pat. No. 5,248,199, respectively, which are both incorporated herein by reference. A conventional heat flux differential scanning calorimeter (DSC) measures the heat flow into and out of a sample with respect to a reference. Both sample and reference are usually subjected to a linear temperature/time ramp. In one implementation of MDSC, a sinusoidal modulation is superimposed on the underlying heating ramp to generate a corresponding sinusoidal response in the heat flow signal. This results in two measurements of heat capacity, an underlying linear long-period measurement due to the underlying heating ramp and a higher-frequency cyclic measurement due to the superimposed sinusoidal modulation. For many systems the cyclic measurement "sees" only the reversible heat capacity associated with molecular vibrations, e.g., glass transitions, whereas the underlying measurement also sees endotherms and exotherms associated with kinetically-controlled events such as recrystallizations, cure reactions or the loss of volatile materials.